Constant torque ferrite slug mounting for inductance adjustments



J1me 1970 H. CURTIS m, ETAL 3,517,363

CONSTANT TORQUE FERRITE SLUG MOUNTING FOR INDUCTANGE ADJUSTMENTS Filed March 18, 1968 2 Sheets-Sheet 1 FIG. IC

' H. CURT/$.HZ' INVENTORS G. A TUQHEN aga Z I ATTORNEY June 23, 1970 H. CURTIS m, ETAL 3,517,363

CONSTANT TORQUE FERRITE SLUG MOUNTING FOR INDUCTANCE ADJUSTMENTS Filed March 18, 1968 2 Sheets-Sheet 2 United States Patent "ice 3,517,363 CONSTANT TORQUE FERRITE SLUG MOUNTING FOR INDUCTANCE ADJUSTMENTS Hazen Curtis, 1H, Andover, and Gerd A. Tuchen, Boxford, Mass., assignors to Bell Telephone Laboratories, Incorporated, Murray Hill, N.J., a corporation of New York Filed Mar. 18, 1968, Ser. No. 713,622 Int. Cl. H01f 21/06 Us, 01. 336-136 6 Claims ABSTRACT OF THE DISCLOSURE A constant torque adjustment of a ferrite slug in an inductor is achieved with an elastic interference fit between the threaded slug and a threaded molded plastic housing. The basis for control of the adjustment torque is that the housing is stretched into a multilobed shape by the interference fit. Both the static and dynamic torque achieved in the structure are highly controlled and near constant over the adjustment range. When the ferrite slug is encapsulated in plastic, the material combination of nylon against acetal polymer provides smooth tuning without stick-slip.

This invention relates to electrical inductors and more particularly concerns a method for achieving a controlled and relatively constant static and dynamic torque over the adjustment range of a ferrite slug.

BACKGROUND OF THE INVENTION There currently are two methods of providing an ad justment track for repeated smooth tuning of a ferrite slug within an inductor core. In one, the clearance between a threaded sleeve and a ferrite slug is filled with a silicone grease which supplies the adjustment torque and holding force. The grease, however, cannot support static loads, which makes it unsuitable where movement even in the order of the clearance of the slug with respect to its sleeve cannot be tolerated. The second method makes use of a differing pitch between sleeve and slug threads to achieve an elastic interference fit. The required close tolerances between mating parts, however, are diflicult and expensive to achieve in practice; and the result often is uneven functioning.

Additional drawbacks of the above methods are the abrasive wear caused by the ferrite material, and their susceptibility to climatic changes.

Accordingly, one object of the invention is to increase the life span of inductors with ferrite slug inductance adjustments.

Another object of the invention is to eliminate the dependence of a ferrite slug adjustment upon close tolerances between the slug and sleeve threads.

An added object of the invention is to effect a near constant static and dynamic torque over the entire adjustment range of a ferrite slug.

SUMMARY OF THE INVENTION In acordance with the basic inventive concept, the ferrite slug is accommodated in an interference fit between its threads and those of like pitch molded into internal vertical or near-vertical flutes of a molded plastic housing. Three such flutes constitute the optimal number, although more may be employed. The interference of the adjustment slug with the threaded flutes stresses the plastic housing into a trilobed shape. The force applied to the threaded slug as a result of the stretching of the housing is the basis for control of the adjustment torque.

In one embodiment, the housing is a vertical sleeve supported by a leg within the interior of a ring mount- 3,517,363 Patented June 23, 1970 ing. The housing thus is free to expand into the desired multilobed shape.

A variation of the inventive concept involves unthreaded flutes within the plastic housing, which are threaded by the advancing ferrite slug. In this instance, the slug is unencapsulated; but in most of the embodiments to be described, the slug is encapsulated in plastic.

One feature of the invention, accordingly, involves the mounting of a ferrite slug within an expandable compliant member such as a molded plastic housing, whose compliance is the vehicle for generating an enduring constant adjustment torque.

The invention, its further objects, features, and advantages will be more fully apprehended from a reading of the description to follow of an illustrative embodiment.

DESCRIPTION OF THE DRAWING FIGS. 1A, 1B and 1C are schematic perspective views of a first embodiment of the inventive sleeve and slug housing arrangement;

FIGS. 2A and 2B are schematic perspective views of a second embodiment;

FIGS. 3A and 3B are schematic perspective views of a third embodiment;

FIGS. 4A, 4B and 4C are schematic perspective views of a fourth embodiment;

FIG. 5 is an exploded assembly of an inductor incorporating the invention; and

FIG. 6 is a frontal perspective view of the assembled inductor of FIG. 5.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT FIG. 1A depicts a molded housing designated 1, which consists of a base ring 2 having a circular recess 2a and an elongated close-sided cylindrical sleeve 3. The interior of sleeve 3 is provided with a number of vertical flutes 4 which, as shown, advantageously are three in number. Flutes 4 in this embodiment are threaded their entire length. Sleeve 3 is attached in integral fashion by a single arm 5 to the base ring 2. Sleeve 3 is less in diameter than the inside diameter of circular recess 2a, so as to provide a substantial clearance therebetween.

FIG. 1C depicts a typical plastic encapsulated ferrite slug assembly which consists of a ferrite slug housing 6, a ferrite slug 7 embedded in the housing 6, a threaded section 8, and a screwdriver slot 9. The material selected as the encapsulant can be any of numerous plastics; and likewise, the material from which the housing 1 is molded can be similarly chosen by those skilled in the art. It has been found, however, that the material combination of a polyamide encapsulant and an acetal polymer sleeve, or vice versa, yields smooth tuning without evidencing any stick-slip characteristics.

The thread size molded into flutes 4 is selected to be a lesser pitch diameter than the threads 8 of slug housing 6, this to assure an interference fit when the slug is placed into the sleeve as shown in FIG. 1B. The threads are of the same pitch. The extent of interference fit depends upon the torque desired; a typical fit is of the order of 10 mils.

FIG. 1B illustrates that the sleeve 3 assumes a trilobed shape from its originally round condition. The stresses in sleeve 3 thus generated are imparted to slug threads '8 by the threaded flutes 4 evenly throughout the area of contact. Alternatively, the sleeve 3 can be molded with the flutes 4 in a trilobed condition which becomes circular when the housing 6 is engaged. Significantly, it is not necessary to closely control the diameters either of the threads 8 or of the threads of flute 4, since it is the elastic deformation existing in sleeve 3 which supplies the resistance to turning of the housing 6. Furthermore, the

assembly of housing 6 and sleeve 3 is less subject to changes in characteristic due to temperature or humidity variations, than the previously-mentioned prior art methods.

In selecting the outer diamter of sleeve 3 and the inner diameter of base ring 2, the inventive concept calls for a sufiicient final clearance between the ring 2 and the lobes 10 of the expanded sleeve 3.

FIGS. 2A and 2B depict a variation of the inventive concept in which the flutes 4 of housing 1 are not prethreaded. In such case, an unencapsulated threaded ferrite slug designated 11, and similar in other respects to slug 6, is used to cut into the unthtreaded flutes 4A. The sleeve 3, however, is deformed in the same manner as portrayed in FIG. 1B.

A further variation of the invention is shown in FIGS. 3A and 3B. Here, the sleeve 3 is provided with but one threaded flute 4b, and further incorporates a flattened vertical portion 12 opposite the flute 4b. Insertion of the threaded stud 6, as shown in FIG. 3, forces the sleeve 3 to assure a near-circular shape; and this deformation applies the radial forces to the slug housing 6.

A still further variation of the invention is shown in FIGS. 4A, 4B and 4C. The sleeve 3 is fully threaded internally. The slug housing 6, however, has three threaded external fluates. When the housing 6 is engaged in the sleeve 3, the latter assumes a trilobed shape as depicted in FIG. 4C.

In all the described embodiments, the primary concern is a well-controlled and near constant static and dynamic torque over the entire adjustment range. This is achieved because, once the threads of the slug are fully engaged into the sleeve 3, the radial forces generated by the sleeve are substantially the same at all points of adjustment of the slug housing 6.

The application of the inventive concept in an inductor structure, is illustrated in FIG. 5. There, the numeral 15 refers to a transformer core and winding assembly consisting of two similar hollow ferrite cup cores l6 and 17 surrounding a winding asembly 18 in a manner that is conventional in the art. A hollow interior along the axes of cup cores 16 and 17 designated 19, is routinely afforded to receive at various adjustments the ferrite slug housing 20 which is substantially identical to the slug housing 6 and slug 7 of FIG. 1C.

Assembly 15 rests on a molded terminal plate 21. A circular recess 22 is provided in the terminal plate 21 to eliminate any bending stresses in the base of cup core 17 due to the clamping force of assembly spring clip 24. The ceneral region of terminal plate 21 is substantially identical to that depicted in FIG. 1A. An assembly spring clip 24 with gripping arms 25 secures the assembly 15 to the terminal plate 21. In this position, the slug housing 20 when engaged upon the threaded flutes 4 is adjusted to various desired settings along the core axis 19.

FIG. 6 depicts the underside of the assembled inductor with the housing 20 adjusted to a position where it is fully engaged Within the flutes 4. The terminals 26 have been connected to the Winding assembly leads 27; and a shielding can 28 is placed over the entire structure. Intended side portions 29 of can 28 engage slots such as 30 molded in treminal plate 23 to secure the can to the plate. The terminal plate rests on internal embossment 31 of can 28.

Tests made on the ferrite slug and threaded sleeve arrangement of FIG. 5 indicate that hnudreds of adjustments can be made in temperature and humidity environments which are typical for inductors without incurring a loss of adjustment torque. This is well in excess of the life expectancy of the inductor or its associated equipment.

It is to be understood that the embodiments described herein are merely illustrative of the principles of the invention. Various modifications may be made thereto by persons skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In an adjustable inductor assembly comprising a magnetic core with winding means, an inductance adjustment slug and an axial cavity Within said magnetic core for receiving said slug, the improvements in the means for mounting the slug comprising:

a closed compliant sleeve having an axial passage for receiving said slug; means including the localized distention of said sleeve in response to entry of said slug for effecting an interference fit between said slug and said passage; and

means for rigidly mounting said sleeve with said passage axis coincident with said magnetic core cavity axis, said mounting means including means for accommodating said sleeve distention.

2. The apparatus claimed in claim 1, wherein said sleeve has at least one internal longitudinal flute for effecting gripping contact with said slug.

3. The apparatus claimed in claim 1, wherein said sleeve has three symmetrically spaced internal longitudinal flutes for effecting gripping contact with said slug.

4. Apparatus in accordance with claim 3, wherein said longitudinal flutes and said slug are threaded with like diameter and like pitch threads.

5. Apparatus in accordance with claim 4, wherein said sleeve mounting means comprises a base having a circular recess and a single leg extending into said recess connecting said base and said sleeve, the undistended sleeve outer diameter being less than said recess diameter.

6. Apparatus in accordance with claim 1, wherein the materials of the contacting surfaces of said sleeve and said slug are respectively acetal polymer and a polyamide.

References Cited UNITED STATES PATENTS 2,995,719 8/1961 Osborn et a1. 336-136 3,191,131 6/1965 Adams et a1.

3,223,952 12/1965 Slenker 336-136 3,227,980 1/1966 Roser 336-136 3,281,744 10/1966 Melanson 336136 XR 3,368,177 2/1968 Hilgers 336-136 XR OTHER REFERENCES Engelmann, German app. No. 1,087,192, pub. Aug. 18, 1960.

Hass, German app. No. 1,142,208, pub. Jan. 10', 1963.

THOMAS, J. KOZMA, Primary Examiner US. Cl. X.R. 

